[X] Contemplation
Heading off the Fae is just insane.
It puts us at serious personal risk, so that we can gain less of the things we came here for and need to get out of this mess in a good shape to act against Anys Syn.
So the decision should only really be between the siege and the prediction.
While the immediate hit to moral from Contemplation could hurt a lot, it is however noteworthy that Uly isn't that good a commander, meaning the actual effects of preparing everyone for battle wouldn't be that impressive and while the moral loss would hurt for this battle, Uly's personal prowess will remain just as effective and our efforts will profit from the prediction bonuses and potentially massively devastating stratagems, so I hardly think our odds to win the battle would be any lower.
And after the battle things turn around again.
The troops will still get to see Uly being an unbelievable murder machine and Rihaku already pointed out that the long term effects of the mystique this course of action creates for our reputation and the loyalty of our forces can easily surpass the advantages of starting preparations off in a more traditional manner.
One gets them to see Uly being a dedicated and competent (if not particularly amazing or inspiring) commander and see him murder the shit out of lots of Fae. The other lets them see him do some confusing and even worrying stuff that ultimately results in absolutely devastating the Fae in a manner noone could have predicted and they still get him murder the shit out of lots of Fae.
Without social or war charms, we are unlikely to benefit nearly as much from trying to establish ourselves as a more traditional commander of any sort, so going with the stuff so ridiculous our subordinates cannot even follow what we're doing ought to work better, especially since this makes it easier to do things that we don't want them to be able to follow.
And that's not even going into the longer term advantages the prophecy will offer afterwards.
The effects of this prophecy would continue to help us later on, which should do a lot to sooth worries that other factions will be abel to take advantage of our legion being exhausted from the Fae and large scale effects like only a prophecy can produce could very well ultimately do more good than trying to save some of those civillians would, especially since we'll always have to choose between going out and killing stuff, whipping our troops into shape or prophesiting from now on.
This could very easily be a situation where making the prophecy has less of an opportunity cost than every again, because the porphecy will directly help with adressing the current threat. There's no saying when we'll get the next opportunity to do something that can help throughout our Luseng adventure.